Sweet Corn Insect Pest Updates

Bill Hutchison, Dept. of Entomology, University of Minnesota, St. Paul, Minn.

European corn borer -- ECB moth catches in light traps remained low to zero this past week at most “bivoltine” areas of southern Minnesota (see graph below). However, as indicated last week, Tom Rabaey with Pillsbury confirmed that the univoltine strain (from west-central to central Minn.) continues to be active, at approx. 25-30 moths/night in the area west of Glencoe. We are now at 1400 degree-days (from Jan. 1 to July 18, 2000) for much of southern Minn., indicating that the 2nd generation flight of our dominant bivoltine strain should be starting in the next 5-10 days.

Corn earworm -- CEW counts also remained low this week in both pheromone and light traps. In most years (including this one), in southern Minn., the dominant CEW flight does not begin until Aug. 20th (+/- 5 days). What has been collected to date, is approx. 0-4 moths/week at all reporting sites; again this is typical for this time of year.

Some processors, including Brian Flood, Del Monte, in IL (where the CEW flight is more active), have noted a problem with some CEW pheromone lures, that do not seem to be working, even with the best lures. We have not observed a problem with our Minn. traps, but will attempt to check this out. All volunteers using pheromone traps for CEW should be using the Hercon “Zea Lure-Tape” product (do not use the Scentry lure). If you believe the pheromone trap is not working in your area, light traps can be used as a presence/absence indicator, i.e., light traps may not catch high numbers of moths, but will usually pick up the beginning of a flight; the presence of just a few CEW moths in a light trap can indicated significant CEW pressure. The bottom line with CEW, for estimating when to apply the first spray OR whether to tighten up a spray interval, should still be to check fresh silks for presence of eggs, and use the low threshold of 4% of the plants infested with eggs or 1st instar larvae (tassel to silk stages).

Flea beetles -- There have been some reports of a new flush of flea beetles in some Minn. sweet corn fields; this is likely a 2nd generation flight. Although susceptible varieties can still be vulnerable to wilt and possible yield loss, I do not believe we have a significant risk of yield/quality losses for the following reasons: a) initial risk, based on overwintering potential was low to moderate (avg. temp. of 22F for Dec. to Feb.), older plants, beyond the V5 seedling stage can tolerate more damage, and/or damage is limited to the leaf blight phase (see table)

The following table, based on research from the Univ. of Illinois, summarizes the reduced risk of yield loss, as a function of plant maturity.

	Potential Yield Losses (%) from Stewart's Wilt (Based on Time of Infection)
Hybrid Reaction	3-5 Leaf Stage	5-7 Leaf Stage	7-9 Leaf Stage
Resistant	0	0	0
Moderately Resistant	0-30	0	0
Moderately Susceptible	10-40	0-10	0
Susceptible	40-100	15-35	3-10

For more information on flea beetlew injury, see the article by Rick Weinzierl, Vegetable Insect Pest Update, March 3, 2000 issue of the Illinois Fruit and Vegetable News, at: http://www.aces.uiuc.edu/ipm~/news/frveg0003.html

Co-Editors: Bill Hutchison, Department of Entomology, University of Minnesota, hutch002@tc.umn.edu
Jeanne Ciborowski, IPM Program, Minnesota Department of Agriculture, jeanne.ciborowski@state.mn.us
Cindy Tong, Department of Horticulture, University of Minnesota, ctong@extension.umn.edu
Production Editor: Suzanne Wold, Research Specialist, University of Minnesota, woldx018@tc.umn.edu

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Last Revised July 20, 2000.
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